The preferred embodiment relates generally to a strategy for controlling a motor vehicle. More particularly, it pertains to electronically detecting potential inertial transfer case engagements in two wheel drive (2WD) mode and manipulating a transfer case clutch to eliminate harshness.
Some four wheel drive (4WD) systems use a ball ramp mechanism to transmit rotary power to a secondary driveline while continuing to transmit power to a primary driveline. Such drive systems are intended to be activated electronically, but sudden accelerations of the input shaft can activate a torque transfer mechanism due to inertial effects. If the system is operating in 2WD mode, this activation can cause a rapid acceleration of the secondary driveline, resulting in a sharp, objectionable noise, called clunk.
During tip-ins from a coasting state at higher speeds while operating in 2WD mode, the transfer case clutch may inadvertently engage and cause a significant clunk. Addressing the clunk through mechanical methods can decrease torque capacity of the 4WD system, increase parastic losses, and/or make the system slower and more difficult to control. Alternatively, increasing the amount of drag in the transfer case clutch system to prevent the secondary driveline from slowing enough to produce a clunk necessarily increases parasitic losses and decreases vehicle fuel economy.
There is a need, therefore, to anticipate the likely occurrence of a clunk in a transfer case due to this cause, to detect electronically the conditions under which such a clunk is likely to occur, and to take corrective action to avoid its occurrence without compromising vehicle fuel economy, diminishing control responsiveness, or increasing parastic losses.